Conventional pneumatic actuators include single-acting actuators and double-acting actuators. A pneumatic module and/or a control module may be operatively coupled to the actuator to assist the actuator in operating and/or controlling a valve to which the actuator is operatively coupled. The pneumatic module and/or the control module may be mechanically coupled to and/or integrated with the actuator.
A single-acting actuator includes a drive shaft that moves in a first direction when a pressurized control fluid is supplied to a port of the actuator. A return spring included in the single-acting actuator causes the drive shaft to move in a second direction opposite the first direction when supply of the pressurized control fluid to the port of the actuator fails and/or ceases. The supply of the pressurized control fluid to the port of the actuator may fail and/or cease, for example, as a result of a pneumatic failure and/or an electrical power failure experienced by the actuator, the pneumatic module and/or the control module. The single-acting actuator may have a fail to close failure configuration or a fail to open failure configuration. In the case of a fail to close failure configuration, the return spring causes the drive shaft of the actuator to move in a direction that results in the valve to which the actuator is operatively coupled assuming a closed position. Conversely, in the case of a fail to open failure configuration, the return spring causes the drive shaft of the actuator to move in a direction that results in the valve to which the actuator is operatively coupled assuming an open position.
In contrast to a single-acting actuator, a double-acting actuator includes a drive shaft that moves in a first direction when a pressurized control fluid is supplied to a first port of the actuator and in a second direction when the pressurized control fluid is instead supplied to a second port of the actuator. The drive shaft of the double-acting actuator maintains its current position when supply of the pressurized control fluid to both of the first and second ports of the actuator fails and/or ceases. Accordingly, the double-acting actuator has a fail in last position failure configuration.